1. Field of the Invention
The present invention relates to a ductile particle-reinforced amorphous matrix composite and a method for manufacturing the same. This composite includes a mixture consisting of an amorphous phase powder and a ductile metallic powder dispersed into the amorphous phase powder.
The mixture is plastically worked by a hot extrusion or a hot forging, and is thereby consolidated. The consolidated products contain small amount of micro-voids and show enhanced inelastic elongation and fracture toughness, compared to those of the monolithic. Further, with this composite structure, the amorphous material can be fabricated to be bigger and versatile in size, thereby manufacturing large-sized products with high quality and high strength.
2. Description of the Related Art
Usually, amorphous materials exhibit high mechanical strength at temperature below a glass transition temperature. For example, Ni-, Ti- or Zr-based amorphous alloy shows the level of fracture strength approximately 2 GPa, and Al-based amorphous alloys show that around 1 GPa. This high fracture strength mainly results from a unique atomic structure of the amorphous material. Therefore, the amorphous material has a great potential in useful engineering applications.
However, the above-mentioned alloys having an excellent glass forming ability are limited in size to be produced. That is, in producing by solidifying the molten alloy into a solid state, the structure of these alloys becomes to be amorphous in a comparatively low cooling rate condition such as 1-250 K/s. However, a maximum size with the amorphous structure attainable by this method is around 10 mm in diameter. Further, the amorphous material shows little inelastic ductility below the glass transition temperature. Although the amorphous material has some plasticity, it deforms with the formation of shear band and strain-hardening behavior does not occur during deformation, then being catastrophically failed. (A. Inoue, Prog. Mat. Sci., 43, (1998), 365)
In order to overcome one of the problems of this size limit, U.S. Pat. No. 4,523,621 discloses a method for making amorphous powder and consolidating this powder by a hot extrusion. Powders are made by a gas atomization method under the rapid solidification condition. Amorphous powder selected from them is contained in a Cu container and sealed. Then, the amorphous powder is consolidated beyond the amorphous transition temperature by a hot extrusion or a hot forging to obtain a bulk amorphous material without size limitation.
In this ""621 method, it is sometimes difficult to consolidate the powder under the condition of maintaining the vitreous state. That is, in order to prevent crystallization in the amorphous alloy, extrusion ratio needs to be reduced. Furthermore, an oxide layer generally formed on the surface of the amorphous powders can reduce the bonding strength between the amorphous powders. Due to these disadvantages mentioned above, the product contains micro-voids between the particles.
In order to prevent the formation of the oxide layer, the entire fabrication processes should be carried out under an Ar gas or vacuum condition, thereby increasing the production cost. Further, after extrusion, the produced sample should be rapidly cooled to prevent crystallization.
In general, the amorphous materials show a catastrophic failure without inelastic deformation. Therefore, there requires a need for making a material for preventing the crack propagation.
In order to solve this fracture toughness problem, various ways have been introduced. For example, there are an amorphous matrix composite made by adding metal powder into a molten alloy and rapidly solidifying the mixture (R. D. Conner, R. B. Dandliker and W. L. Johnson, Acta Mater., 46 (1998) 6089), a composite made by penetrating a molten alloy into dispersing tungsten wires and cooling the mixture (U.S. Pat. No. 6,010,580) and a composite, on which a ductile phase is first formed by controlling the solidification route then the other becomes an amorphous phase (C. C. Hays, C. P. Kim and W. L. Johnson, Proc. ISMANAM. ISMANAM-99, Mater. Sci. Forum, Dresden, Germany, 2000). All these cases relatively improve inelastic elongation, but form amorphous phase at the time of solidification of the molten alloy, thereby limiting the produced size.
Accordingly, an object of the present invention is to improve the above-described conventional problems such as size and/or shape limit and fracture toughness.
Another object of the present invention is to provide a composite, in which ductile metallic particles are dispersed in an amorphous matrix, and a method for manufacturing the same. Herein, the composite is manufactured by mixing ductile powder and amorphous powder in a predetermined volume fraction of ductile powder and extruding or forging the mixture beyond the amorphous transition temperature and below the crystallization temperature (i.e., in the range of super-cooled liquid region). Thereby, both the amorphous powder and the ductile powder are plastically deformed and consolidated each other.
In order to achieve the foregoing and other objects, the present invention provides a ductile particle-reinforced amorphous matrix composite characterized in that a ductile powder is dispersed in an amorphous matrix made by an amorphous powder.
The amorphous powder includes one alloy powder which can be produced in the form of amorphous phase, for example, Ni-, Ti-, Zr-, Al-, Fe-, La-, Cu- or Mg-based alloy.
The ductile powder includes any metallic alloy with a flow stress lower than that of the amorphous powder during the fabrication in the super-cooled liquid region.
In the super-cooled liquid region, the amorphous material deforms via viscous flow and the ductile powder is strained more than that of the amorphous material.
Herein, the level of stress of the ductile powder should be lower than that of the amorphous powder. In case of using the ductile powder with higher stress, the ductile powder is not deformed and remains with an initial shape, or is strained less than the amorphous powder, thereby reducing the interfacial bonding strength between the ductile particles and the amorphous particles or forming micro-voids between the interfaces. This deteriorates the mechanical properties of the composite.
The content of the ductile powder is designated as a predetermined range for improving inelastic elongation without significantly losing the strength of the composite, compared to that of the material including only the amorphous powder.
In order to obtain this object, the ductile powder is preferably 0.1 vol % through 40 vol %.
Since the ductile powder with a content of more than 50 vol % makes the composite the ductile matrix, the ductile powder is contained in less than 50 vol %.
Usually, if the ductile powder is more than 30 vol %, the aggregation among the ductile particles occurs. Therefore, the added ductile particles should be isolated from each other and dispersed randomly into the amorphous powder.
However, the upper limit of the ductile powder of the present invention is 40 vol %. As shown in FIG. 5, the ductile powder with a content of 30 vol % does not particularly show the aggregation. Further, the lower limit of the ductile powder of the present invention is 0.1%. The ductile powder with content less than 0.1 vol % does not provide our objectives.
Further, since the ductile powder is selected from any material with a stress lower than that of the amorphous powder in the super-cooled region during the fabrication, the ductile powder is not limited in an particle shape, i.e., fiber or spherical shape and in an particle size.
In another aspect of the present invention, a method for manufacturing a ductile particle-reinforced amorphous matrix composite is provided. The method comprises steps of preparing a mixture consisting of amorphous powder and ductile powder; obtaining a billet by compacting said mixture in a hermetically sealing condition; and plastic working the billet at a super-cooled liquid temperature range of the amorphous powder.
The billets are plastically worked by a hot extrusion or a hot forging. Herein, the amorphous particles do not transform to be crystallized and remain the amorphous phase.
The amorphous matrix composite is manufactured as a final product by mechanically machining, electric discharge machining or forming at the super-cooled liquid temperature.
The amorphous matrix composite manufactured according to the present invention includes ductile powder, thereby reducing the formation of micro-voids which are generated in the conventional method for the material including only the amorphous particles. Since the ductile powder serves as a barrier for propagating the shear band or crack as well as a starting point of the shear band formation, the composite of the present invention provides the improved inelastic elongation and fracture toughness at a room temperature.